Freezing Time Predictions for Different Final Product Temperatures

A modification to an empirical food freezing time prediction formula is proposed that allows the formula to be used for a range of final product temperatures. Over a large data set (275 runs) the percentage difference between experimentally measured times and predictions has a mean of 0.2% and a standard deviation of 6.8% for the improved formula compared to -2.4% and 8.5%, respectively, prior to modification. Ninety percent of the predicted freezing times by the new method were within ± 11% of the experimentally measured times and ± 9% of predictions by an accurate finite difference scheme. This performance compares favorably with other published freezing time prediction methods. The same type of modification for varying final product temperature may be suitable for other empirical formulae.     

A Simplified Model for Freezing Time Calculations in Foods

A model is proposed for freezing time calculations which combines Plank's equation with the unsteady heat transfer solutions for the cooling of a slab of constant properties through the addition of pre-cooling, change of phase and tempering periods. The change of thermal properties with the ice content is taken into account by proposing average values for the different periods. No adjustable parameters are used in developing the model. Results are compared for the case of beef freezing with those obtained numerically by using a heat transfer model with simultaneous change of phase and with experimental measurements showing good agreement.     

食品冷冻技术的研究进展

冷冻技术的发展异常迅速,在食品工业中的应用也越来越广泛,将其更好地应用于食品中成为当前研究者较为关注的课题。重点对冷冻过程中的理论研究及其在食品工业中的应用进行综述,并简述了近年来国内外食品冷冻技术的现状与发展趋势。     

3种速冻甘薯食品的加工工艺

主要介绍了出口速冻甘薯、冷冻甘薯片和速冻甘薯茎尖的生产工艺和操作要点，以期为生产厂家起到一定的参考作用。     

A Method to Determine Initial Freezing Point of Foods

An accurate initial freezing point T_sh is required for reliable analysis of a food freezing or thawing process. Two previous formulae for accurately estimating enthalpies, one for temperatures above T_sh and another below T_sh, were used for this development. The estimation of T_sh was based on the mathematical formulation of continuity in food enthalpies estimated by both formulae at T_sh. The method was validated by estimating T_sh values of aqueous solutions of sucrose and NaCl at different solutions. There was fair agreement in the T_sh values of fresh fruit and vegetables, lean beef meat and lean fish meat estimated by the proposed and a previous method. One potential advantage of our new method was the direct estimation of T_sh through analysis of food enthalpies collected at different temperatures.     

Bacterial Extracellular Ice Nucleator Effects on Freezing of Foods

Extracellular ice nucleators (ECINs) were incorporated into foods and subjected to subzero freezing. Time-temperature profiles, ice-formation patterns and textures were examined by thermocouple, microscopy and texture analyzer. Onset temperatures (initial freezing), enthalpies and freezing rates were measured by DSC. Addition of ECINs to liquid foods elevated ice nucleation temperatures and promoted freezing. Solid or semisolid products frozen with ECINs resulted in a fiber-like texture. These effects were more apparent at –10°C or higher. Differential scanning calorimetry revealed onset temperatures were increased 11°C by addition of ECINs, but length of time to complete the phase transition was extended at constant cooling rates. Results indicated that ECINs can be used instead of whole bacterial cells for efficient freezing and textural modification.     

平板状食品冻结时间的数值预测

采用有限差分法获得平板状食品冻结时间的数值解。对凝固区（两相区）的参数进行处理,使食品在整个冻结过程可以选用统一的差分格式;通过凝固区释放热量的累积效果,判断凝固区界面的移动;对常见的平板状食品的冻结过程进行计算机模拟来预测冻结时间,并通过实验来验证可靠度。结果表明,这种计算方法可以应用在实际生产中。     

Water Activity and Freezing Point Depression of Aqueous Solutions and Liquid Foods

Water activity (a_w) of binary aqueous solutions and liquid foods (skim milk, coffee beverages, peach and tomato juices) at different total solids concentration (by wt).was evaluated, measuring freezing point depression (FPD). The calculated values differed from those obtained by electric hygrometer by less than 0.01 a, units. Water activity was also evaluated in model systems containing ethanol, for which a_w determination by conventional electric hygrometer is not possible.     

Predictions of Microbial Thermal Inactivation in Solid Foods: Isothermal and Non-isothermal Conditions

This work focuses on the use of the Gompertz-inspired model to predict the thermal inactivation behaviour of microorganisms obtained in solid food products, validated for isothermal and non-isothermal conditions. Experiments were carried out in parsley, artificially inoculated with Listeria innocua. For the isothermal conditions tested, the predictive ability of the model was confined. The higher the temperature, the higher deviations observed (i.e. the model underestimates the inactivation behaviour). However, for the non-isothermal condition tested, the model predicted the microbial response accurately.     

Freezing Time Modeling for Small Finite Cylindrical Shaped Foodstuff

A model was developed consisting of a modified Plank's equation to estimate phase change time and two unsteady state cross-product heat transfer equations for estimating precooling and tempering times. It accurately predicted total time to proceed from an initial temperature above the freezing point to a final temperature of ?18°C. A correction factor was developed and incorporated in the P term in Plank's equation to correct for the effect of initial and freezer medium temperatures and heat transfer coefficients. The model, tested over a broad range of freezing conditions, had a mean absolute error of 5.9% in predicted values relative to experimental values.     

Assessment of Freezing Time Prediction Methods

A systematic procedure for testing the accuracy of freezing time prediction methods is discussed. It involves carrying out freezing time predictions by an accurate numerical method as well as the method under test, and careful cross-checking of prediction errors for correlation with a number of physical parameters. Criteria are given for selection of an appropriate numerical method. The need for systematic cross-checking is illustrated by comparisons between four recent prediction methods. The comparisons show the strengths and weaknesses of the four methods. They also indicate general shortcomings in available methods and data, and so highlight those areas in which future research might be most profitable.     

简单形状食品冻结时间计算方法的分析研究

冻结时间对食品的冻结质量和能耗有重要意义.通过对几种简单形状食品冻结时间计算模型和相应公式的分析和计算,对它们在特定工况下应用的准确性和可靠性进行了比较,并对相关计算参数的选取给出了说明和建议.     

A Simplified Analytical Model for Thawing Time Calculation in Foods

A simplified analytical model for the thawing time prediction of simple-shaped foodstuffs was developed. It was assumed in the model that the solution to the unsteady state, unidirectional heat conduction equation with constant thermophysical properties was valid throughout the thawing operation. The latent heat effects were incorporated into effective diffusivity terms defined for the various phases of the thawing operation. The predictions of the model were compared to experimental and numerical data obtained on thawing of infinite slabs, infinite cylinders, and spheres. The level of agreement between the predictions of the present model, and the experimental and numerical data was quite satisfactory.     

果蔬汁等液态食品之冻结点的经验计算

冻结点是果蔬汁等液态食品冷冻加工与保藏中的重要参数，冻结点和冻结点的下降与果蔬汁等液态食品的组成分及浓度有关。本文介绍了几种通过计算冻结点下降值求得冻结点的经验公式。     

我国冷冻、冷藏型面类快餐调理食品发展浅析

０概论快餐调理食品在食品产业中占有非常重要的地位，也是国外食品行业中发展最快的产业。以日本为例：以１９７５年为基准的食品消费额的统计显示，到１９９８年的２２年间整体食物费用仅增长了３５．６％，“外食”增长了５３．８％，而快餐调理食品增长了２４０％。我国大规模地从国外引进各类食品生产设备始于８０年代中期，但限于当时的物流环境和消费水平，这些设备大都以生产单一的、能够长期保存的食品或方便食品为主。近年来随着我国生活水平的提高，工作节奏的加快和旅游业的迅速发展，食品产业结构单一的矛盾已逐渐显现。主食产…     

Parametric Analysis for Predicting Freezing Time of Infinitely Slab-Shaped Food

A computer program was developed in order to predict freezing times of homogeneous food which may be approximated by infinite slabs. For this solution we assume temperature dependent thermophysical properties, together with symmetric or nonsymmetric convective and radiative heat exchange imposed at the boundaries. Through application of a statistical analysis, all dimensionless groups that have a significant influence in the rate of heat transfer were selected and used to develop a regression equation for the prediction of freezing times, by applying a central composite experimental design. This equation is applicable to the wide range of parametric values encountered during commercial operations. The reliability of the computer program and regression equation developed was verified experimentally.     

Freezing Time Prediction: An Enthalpy-Based Approach

Equations were developed for prediction of freezing times of foods of slab geometry, for three different boundary conditions, using a method of solving for enthalpy per unit volume instead of temperature. Experimental verification was performed for the operation with a convective heat transfer boundary condition, using ice cream and green peas in a granular packed bed as test materials. Results indicate good prediction of freezing times over a range of conditions. Results of thawing tests are less satisfactory if the ambient temperature lies in the zone of rapidly changing food properties. A comparison with literature predictions is presented.     

Thermodynamic Analysis of the Freezing and Thawing of Foods: Enthalpy and Apparent Specific Heat

On the basis of the principles of freezing point depression, equations for the temperature rate of ice formation, apparent specific heat, and enthalpy were derived and expressed in terms of solids content and temperature. A unique characteristic constant for each food system can be determined experimentally from the calorimetric measurements. The new equations differ only slightly with similar equationspreviously developed by Bartlett in 1944 and by Schwartzberg in 1976. Reliability and accuracy of the methods are demonstrated with literature data for meat, fish and fruit juices. Enthalpy and apparent specific heat values for a range of temperature between 20° and 40°C and solids content between 10 and 50% are presented.     

Thermodynamic Analysis of the Freezing and Thawing of Foods: Ice Content and Mollier Diagram

On the basis of the freezing point depression equation, an ice content equation was derived. The required parameters are similar to equations for enthalpy and apparent specific heat previously developed. Validity and accuracy of the equation are demonstrated with experimental data for meat, fish and fruit juices. Cohesive data for enthalpy, apparent specific heat and ice content are calculated for a wide range of temperatures between 20 and -40°C. The calculated results are consistent with values obtainable through enthalpy-moisture content-temperature (Mollier) diagrams developed by Riedel.     

Effect of Biot Number and Freezing Rate on Accuracy of Some Food Freezing Time Prediction Methods

Existing food freezing time prediction methods yield widely differing results at high Biot numbers (internal resistance controlling), a situation that could not be previously resolved due to lack of data. Freezing tests were carried out on slabs of Tylose gel in a plate freezer to obtain more data, which alone and in combination with previous data, were used to test the methods of Cleland and Earle, Hung and Thompson, Pham, and finite differences. Good agreement was obtained with Pham's methods and with finite differences. Biot number, freezing rate and supercooling had negligible effect on the accuracy of these methods. The accuracy of the finite difference method has been significantly improved by our new thermal property data for Tylose gel.